The present invention relates, generally, to a method and an apparatus for demodulation, in particular, relates to a method and an apparatus for demodulation of digital information recorded by a PM and FM system on a magnetic or optical recording medium.
The NRZ (non-return-to-zero) system has been utilized for the recording system for digital information. However, the NRZ system has the disadvantages that: (a) the recording frequency changes are large due to the change of the pattern of the recorded information; (b) a particular gate pulse is necessary for the demodulation of the recorded information; (c) the allowable length of skew due to the lateral vibration of recording medium is extremely severe. In order to overcome the disadvantages of the NRZ system, the two-frequency coherent phase system, that is, PM system or FM system, has lately been proposed and utilized very widely in the art. Particularly, the latter system is widely utilized for the recording of a bank card since the American Bankers Association adopted that system. The FM and PM system has no problem related to said skew, since the self-clocking operation in a recording channel is automatically performed, and has many advantages compared with the prior NRZ system.
However, the prior demodulation system for an FM or PM system has a disadvantage that the longitudinal moving speed of a recording medium should be kept strictly constant, and that a complicated mechanism with an electric motor is needed for the movement of a magnetic card.